[PATCH] drm: aux ->transfer() can return 0, deal with it

[Daniel Vetter]

On Thu, Jul 28, 2016 at 05:54:42PM +0300, ville.syrjala at linux.intel.com wrote:
> From: Ville Syrjälä <ville.syrjala at linux.intel.com>
> 
> Restore the correct behaviour (as in check msg.reply) when aux
> ->transfer() returns 0. It got removed in
> commit 82922da39190 ("drm/dp_helper: Retry aux transactions on all errors")
> 
> Now I can actually dump the "entire" DPCD on a Dell UP2314Q with
> ddrescue. It has some offsets in the DPCD that can't be read
> for some resaon, all you get is defers. Previously ddrescue would
> just give up at the first unredable offset on account of
> read() returning 0 means EOF. Here's the ddrescue log
> for the interested:
> 0x00000000  0x00001400  +
> 0x00001400  0x00000030  -
> 0x00001430  0x000001D0  +
> 0x00001600  0x00000030  -
> 0x00001630  0x0001F9D0  +
> 0x00021000  0x00000001  -
> 0x00021001  0x000DEFFF  +
> 
> Cc: Lyude <cpaul at redhat.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch
> Cc: stable at vger.kernel.org
> Fixes: 82922da39190 ("drm/dp_helper: Retry aux transactions on all errors")
> Signed-off-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Seems to correctly restore that special case.

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Dave, since I just send out a drm-misc pull, can you pls apply this one
directly to drm-next?
-Daniel

> ---
>  drivers/gpu/drm/drm_dp_helper.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/drm_dp_helper.c b/drivers/gpu/drm/drm_dp_helper.c
> index 091053e995e5..8f11b8741e42 100644
> --- a/drivers/gpu/drm/drm_dp_helper.c
> +++ b/drivers/gpu/drm/drm_dp_helper.c
> @@ -203,7 +203,7 @@ static int drm_dp_dpcd_access(struct drm_dp_aux *aux, u8 request,
>  
>  		ret = aux->transfer(aux, &msg);
>  
> -		if (ret > 0) {
> +		if (ret >= 0) {
>  			native_reply = msg.reply & DP_AUX_NATIVE_REPLY_MASK;
>  			if (native_reply == DP_AUX_NATIVE_REPLY_ACK) {
>  				if (ret == size)
> -- 
> 2.7.4
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch